A tubular electrostatic precipitator is disclosed, inter alia, in German Pat. No. 21 34 576 wherein the housing of the middle part consists of plastic material, the tube sheets are made of metal which is coated with plastic material, and the cylindrical collecting tubes are suspended from the upper tube sheet and are sealed therein by means of fixing and sealing rings.
While the plastic housing of the middle part does not involve problems regarding corrosion, it leaves much to be desired as regards its mechanical properties.
For instance, in the manufacturing of relatively large housings the required dimensional accuracy involves a high expenditure. Besides, a separate outer supporting system of steel is always required because unless the plastic wall has an economically unreasonable thickness it will not have the strength required to support the bank of tubes and the top part of the electrostatic precipitator. Moreover, plastic material must be expected to gradually deform under load so that the distance between the corona electrodes and the collecting electrodes may be slightly changed and such change adversely affects the operation of the electrostatic precipitator.
Because the collecting tubes are supported by the upper tube sheet, the latter must be sufficiently rigid. For this reason at least the upper tube sheet had to be made of steel and protected from corrosion by a coating of plastic material. Coatings of plastic material can be provided in a relatively easy manner on objects which have a simple shape. But a tube sheet is formed with bores for receiving the collecting tubes and may be provided with reinforcing ribs joined by welding so that the application of an effective coating of plastic material to such a tube sheet involves substantial difficulties.
Besides, damage to the protective coating in transit and during the erection of the precipitator cannot be precluded. Moreover, adapting operations must often be performed on the site and may involve a removal of part of the coating. For this reason, additional coating material must be applied on the site but even this will not ensure an absolutely reliable protection against corrosion. As a result, corrosion may take place which cannot be detected on the surface but in the course of time may result in a failure of the entire plant.
The self-sealing mounting of the collecting tubes in the upper tube sheet does not reliably prevent an uncontrolled ingress of acid-containing gases into the space between the collecting tubes, where such gases may cause various defects.
It has been attempted to avoid some of these disadvantages by the use of the electrostatic precipitators disclosed in German patent specification No. 21 30 074 and 26 41 114 (see also U.S. Pat. No. 4,155,792). In said precipitators, the collecting electrodes define flow passages which are hexagonal in cross section.
While the cross-section of the housing is utilized in an improved manner in this case, the collecting capacity is adversely affected because the distance between the corona electrode and the collecting electrode is not uniform around the periphery and the operating voltage must be selected in dependence on the smallest distance. For this reason only six narrow strip-shaped areas can be optimally adjusted to this distance in a hexagonal collecting tube and a lower efficiency is obtained in the remaining areas.
These electrostatic precipitators also give rise to problems regarding the uniform wetting and the grounding. For instance, in accordance with German patent specification No. 21 30 074 special measures must be taken because the "outside" surface of the collecting electrode is not available for grounding.
Finally, hexagonal tubes or plate strips which define hexagonal passages (in accordance with German patent specification No. 26 41 114) are much more expensive than collecting electrodes consisting of standardized, circularly cylindrical plastic tubes and the assembling of such hexagonal tubes or of such plate strips is also more difficult and more expensive.